1. Technical Field
The present invention relates to a communication system that uses a pulse width modulation (PWM) code as a transmission code.
2. Related Art
Among communication systems that are mounted in vehicles, a system is known that uses a PWM code as a transmission code (for example, refer to SAE International J1850). Here, of the signal levels on a transmission line, a high signal level is recessive and a low signal level is dominant. The transmission line is configured so that, if even one node outputs a dominant signal, the signal level on the transmission line becomes dominant. In this instance, a configuration may be considered in which this feature is used to enable the waveform of a transmission code to be determined by the signals outputted by each node being overlapped.
In other words, a PWM code having a small low-level ratio is associated with logic 1. A PWM code having a large low-level ratio is associated with logic 0. In a bus-idle state in which no node is communicating, one (master node) of the nodes outputs a PWM code of logic 1. The nodes (slave nodes) other than the master node each output a signal that, when overlapped with the PWM code of logic 1 outputted by the master node, causes the transmission code on the transmission line to become a desired PWM code.
Specifically, for example, when a slave node outputs a PWM code of logic 1, the PWM code of logic 1 is produced by a high-level signal being outputted over the overall period of the code. In addition, when a slave node outputs a PWM code of logic 0, the PWM code of logic 0 is actualized by a signal being outputted that rewrites a portion of the PWM code of logic 1 outputted by the master node from a high signal level to a low signal level.
A driver circuit of the node is ordinarily configured using a transistor that enables and blocks conduction between a transmission line and a ground line. In other words, when the output of the own node is recessive, the node turns OFF the transistor. When the output of the own node is dominant, the node turns ON the transistor.
Therefore, when the slave node rewrites the PWM code to the PWM code of logic 0 as described above, the driver circuit of the master node switches the output thereof from a low signal level to a high signal level while the output of the driver circuit of the slave node is held at a low signal level. Then, in that instant, the current flowing to the driver circuit of the master node flows into the driver circuit of the slave node that has rewritten the code (in other words, that is outputting the low signal level). As a result of the sudden change in current, a problem occurs in that significant noise is generated.